1. Field of the Invention
The present invention relates to a data recording apparatus which records data on nonvolatile, semiconductor-based recording medium.
2. Description of the Related Art
Conventionally, semiconductor memory devices such as an optical disc, a magnetic disc, and a flash memory have been used as a storage memory for data recording. Such a storage memory has an increasing difficulty in realizing high capacity and high speed in the future. Accordingly, a good alternative with small size, high capacity, and high speed as well as low cost has been desired to be put into practical use.
Under this background, it has been expected to put a data recording apparatus using semiconductor-based recording medium with the Metal Nitride Oxide Semiconductor (MNOS) structure which allows recording, reproducing and erasing of data, and a long-term storage of the recorded data, into practical use as one possibility of future data memory devices with high capacity and high speed. As such a data recording apparatus, a semiconductor disc memory device is disclosed and proposed in JP-A No. S55-150268 (KOKAI), for example. In this semiconductor disc memory device where a first insulating film is formed on a semiconductor substrate and a second insulating film having an electric charge accumulating function is formed on the first insulating film, a recording electrode to which a recording signal voltage is applied is made to relatively shift, and an electric charge corresponding to the recording signal voltage is accumulated to the second insulating film through the first insulating film, so that data recording is performed.
Specific structure and function of such a data recording apparatus will be briefly described below. A conventional data recording apparatus using the MNOS medium includes a recording medium that consists of a semiconductor substrate which is made of an n-type silicon (Si); a silicon oxide film (SiO2) which is formed on one surface of the semiconductor substrate and is several nanometers in thickness; a silicon nitride film (Si3N4) which is formed on the silicon oxide film (SiO2) and is several tens of nanometers in thickness; a protection film which is made of carbon or the like and formed on the silicon nitride film (Si3N4) and; Fomblin®-based lubricant which is thinly coated on the protection film; and an electrode which is made of aluminium and formed on the other surface of the semiconductor substrate, for example.
The data recording apparatus using the MNOS medium further includes a needle electrode having an insulating support body which is made of sapphire or the like and relatively shifts by floating from or directly contacting with the recording medium, and a metal film which is made of tungsten or the like and deposited on a surface of the insulating support body; and a control circuit for data recording and erasing which is connected to the needle electrode.
In this data recording apparatus, the control circuit for data recording and erasing controls a voltage corresponding to a recording signal to be applied between the needle electrode and the semiconductor substrate via the aluminium electrode formed on the other surface of the semiconductor substrate. Due to a tunnel effect, an electric charge corresponding to the recording signal is accumulated in the silicon nitride film (Si3N4) through the silicon oxide film (SiO2), and thereby the data is recorded.
Since a depletion layer is formed in the semiconductor substrate depending on the accumulated electric charge, using the needle electrode as an electrode for detecting capacitance when reproducing the recorded data allows sensitive reproduction of whether the depletion layer is present or not, i.e., whether the recorded data is present or not.
In this data recording apparatus, a high speed data recording can be realized by applying a positive voltage pulse to the needle electrode. Specifically the positive voltage pulse has an amplitude of approximately several volts to 30 volts and a pulse width of several nanoseconds to several tens of nanoseconds or less. Moreover, at least ten years of secure storage and readout of data under a normal conservation condition is guaranteed.
However, the conventional data recording apparatus as described above, has a problem that a sufficient data erasing cannot be achieved even if a negative voltage pulse is applied to the needle electrode as for erasing the recorded data. Specifically the negative voltage pulse is, for example, 50 volts or more in amplitude and 200 nanoseconds or more in pulse width, which is more than that necessary for data recording.
In the conventional data recording apparatus as described above, data erasing characteristics is significantly inferior to data recording characteristics in speed. Specifically, when the pulse width is short, no matter how large the voltage for data erasing is, it is difficult to erase the data completely. This problem has been a major obstacle for realization of the data recording apparatus, and an established technology for solving this problem has been strongly desired.